Various industries and processes, such as combustion, cement production, and asphalt production require high temperature filtration to remove undesirable particulate matter from a fluid stream or capture the fine particulate product from a fluid stream. One example of a filtration device used in such industries is a baghouse filter. Most high temperature filtration applications employ fabric filter media at operating temperatures ranging from about 135° C. to about 200° C. Traditional dust filtration materials are made from woven or nonwoven media. Newer filters, including higher surface area pleated media, are made from spunbond or other nonwoven media such as needle punched or hydro-entangled nonwoven technology, for example. Filter media capable of being used at these high temperatures include media produced from polyphenylene sulfide (PPS) (e.g., Torcon®, from Toray Fibers, Osaka, Japan or Procon®, from Inspec Fibers, Lenzing, Austria) or aramid based fibers, (e.g., NOMEX®, from Dupont, Wilmington, Del.). The chemistry of the gas stream to be filtered dictates whether PPS or aramid is used.
The filter media can be pleated to increase the effective filtering area while occupying the same or less space in a baghouse. However, conventional polymer filter media must be treated with stiffening agent resin systems to impart the necessary features to allow the media to pleat and retain its form and functionality at the desired application temperatures. For instance, PPS based filter media without stiffening resin becomes soft and loses pleat definition at the high application temperatures due to PPS's low glass transition temperature (Tg) of 90° C. This loss of pleat definition renders the filter nonfunctional.
Thus, stiffening resins are impregnated into filter substrates to strengthen and stabilize the filter media for use at elevated operating temperatures. The resins are impregnated into the filter media by a multi-step secondary process, wherein the filter media are immersed in a bath of resin solution and then nip squeezed to remove the excess solution prior to drying. After drying the resin, the filter media are rendered stiff.
Yet, conventional stiffening resins, epoxies, or phenols are not fully cross-linked or cured when initially dried onto the fabric. This may allow the media to re-soften during subsequent high pressure, high temperature processes, including during the filtering operation, wherein the filter media does not fully cure and re-stiffen for up to several hours. Once fully cured, the filter media is able to withstand the filtering process. However, pleat collapse or pinching can occur while the material is soft prior to curing, which can result in re-stiffening of the filter medium in a collapsed state. If such a collapse and re-stiffening occurs while the filter media is being used in a high pressure, high temperature filtration process, the collapsed filter media becomes partially or completely nonfunctional.
Previously, polyamide-imide (PAI) resins have been successfully used to render aramid filter media stiff. See U.S. Pat. No. 6,752,847, the disclosure of which is expressly incorporated herein by reference in its entirety. Though a suitably stiff high temperature filter media can be produced, resins and a complicated, multi-step process must be used, both which are expensive. The cost of the stiffening resin can become prohibitive, as the resin is the most expensive material (on a weight per unit area basis) used to make pleated filter media. Given that a pleated element usually uses 2.3 to 3 times the amount of filter media in an equivalent sized traditional filter bag, the amount and cost of the stiffening resin used becomes critical. The amount of stiffening resin required is proportional to the amount of filter media used, and therefore, the cost of the stiffening material becomes significant.
Accordingly, there is a need for a simple and economically desirable filter medium that will be pleatable and remain rigid and functional when used in high temperature filtering applications and a simple and economical method for producing the same.